yes you want to make sure the 15 is rated at 8ohms and you will need to switch the 2x10 to 8ohms. If the 15 is 4 ohms, then you can only plug the 15 in without the 2x10. I have an ampeg B4R and it goes down to 2ohms but im not sure if the svt3 does or not. I im not sure what id do about gettin the horn or not. If its a huge price different i wouldnt. If your gonna run it with the 2x10 all the time i wouldnt. The only thing is if you run the 15 by itself you might want the horn for little extra highs. You can always turn one off in the back of the cab. And your right mars volta does own.

the two horn thing is subjective. i am fine with no horns. if you are a guy who likes it high (that rhymes) then i guess 2 horns is good for you. if you want to give yourself the option of just using the 15 alone, then id get the horn with it. like he said, you can always turn the horns down in the back if you feel the need. hope this helps.